The invention relates to a holographic recording-reading process and a device for implementing this process.
In known holographic recording-reading devices, the hologram of an object is recorded from an object wave diffracted by the object, the hologram of which it is desired to record, and from a corresponding wave. Materials are at present available for writing phase networks or holograms with high diffraction efficiency such as dichromate gelatine, or for operating in real time with such materials as electro-optical crystals, for instance bismuth-silicon oxide. These materials are "thick" with respect to the mean pitch of the photoinduced strata, and their behavior on writing and reading is described by the formalism of the coupled waves; the angular selectivity Bragg effect may be more especially used.
The reading of holograms is achieved by illumination with coherent optical radiation, e.g. by means of the reference beam which was used for recording. So that this reading does not cause erasure of the written hologram, the previously induced spatial index variation should not be substantially modified by such radiation.
If the object wave is complex and thus contains a set of non-colinear wave vectors, there is formed in the photosensitive support medium a spectrum of resulting vectors having different lengths and orientations. Within the scope of the known art it is then not possible to diffract an image under optimum conditions if the reading wavelength is different from that used for holographic recording in the thick material. This is an important restriction. In fact, for information recorded by photorefractive effect in electro-optical crystals of the bismuth-silicon oxide type (BSO), re-reading at the same wavelength erases the information by relaxation of the space charge field.
If it is a case of non-destructive reading, it may be interesting to read with visible radiation a recording made with smaller wavelengths. Thus, the optical components on dichromate or bleached gelatine have a limited spectral range which goes from about 0.23 micrometers to about 0.58 micrometers, which considerably limits use thereof. The recording method proposed in the present invention provides re-reading, under Bragg conditions, by means of a wavelength chosen outside the spectral sensitivity range of the bismuth-silicon oxide (BSO) type crystals for example.